"Breastfeeding represents a modifiable factor that could prevent some cases of this breast cancer subtype and reduce the number of African-American women dying from this disease," says Prof. Julie Palmer.
The researchers, led by Prof. Julie Palmer from Boston University's Slone Epidemiology Center (SEC), MA, publish their results in the Journal of the National Cancer Institute.
They note that women who have had children (parous women) have an increased risk of developing estrogen receptor-negative breast cancer, but because childbearing patterns differ by race and ethnicity - with higher parity and a lower rate of breastfeeding in African-American women - the team studied the link between parity and lactation to the risk of certain breast cancer subtypes.
"Breast cancer mortality is disproportionately high in African-American women of all ages, in part due to the higher incidence of estrogen receptor-negative breast cancer, with fewer targets for treatment," says Prof. Palmer.
According to the Triple Negative Breast Cancer Foundation, these subtypes of the disease are diagnosed from the lack of three receptors known to encourage certain breast cancers: estrogen receptors, progesterone receptors and human epidermal growth factor receptor 2 (HER2).
In women with triple-negative breast cancer, all three of these receptors are absent. Though this type of breast cancer can respond to chemotherapy, triple-negative tumors do not typically respond to receptor-targeted treatments.
Additionally, depending on the stage, this type of cancer can be especially aggressive and is more likely to recur than other subtypes.
'Parous women who have not breastfed at increased risk'
To further investigate factors contributing to breast cancer subtypes in African-American women, researchers from SEC partnered with colleagues at Roswell Park Cancer Institute of Buffalo, NY, and the University of North Carolina Lineberger Cancer Center.
Together, they combined data from four large studies on breast cancer cases, which included the Boston University Black Women's Health Study.
In total, the studies included 3,690 African-American women with breast cancer, 1,252 of whom had the estrogen receptor-negative subtype.
Fast facts about triple-negative breast cancer
- Around 75% of patients with a BRCA1 mutation who develop breast cancer have the triple-negative type
- African-American women who develop breast cancer have a 20-40% chance of it being triple-negative
- Pre-menopausal, African-American, Latina and Caribbean women have an increased risk of developing basal-like or triple-negative breast cancer.
The researchers observed that women who had children were faced with a 33% higher chance of developing estrogen receptor-negative breast cancer than women who had never given birth.
And women who had four or more births but who had never breastfed any of their children had a 68% higher chance of developing this breast cancer, compared with women who had only one birth and who had breastfed.
The team notes that while previous research has suggested that overall breast cancer risk rises during the first 5-10 years after giving birth - with a subsequent risk reduction - their recent study suggests the risk for estrogen receptor-negative breast cancer remains over time.
Though the biological mechanisms behind this link are unclear, the researchers hypothesize that the immune system or inflammatory processes that occur during the period after giving birth may play a role.
They conclude their study by writing that their findings "suggest that parous women who have not breastfed are at increased risk of [estrogen receptor] and triple-negative breast cancer," and add that the low rates of breastfeeding in African-American women may be partially responsible for their higher incidence of these subtypes of breast cancer.
"Breastfeeding represents a modifiable factor that could prevent some cases of this breast cancer subtype and reduce the number of African-American women dying from this disease," adds Prof. Palmer.
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